2.4. Colormap Dialog

Figure 15.14. An indexed image with 6 colors and its Colormap dialog

The Colormap (Indexed Palette is a better name) dialog allows you to edit
the colormap of an indexed image. (If the mode of the active image is RGB
or Grayscale instead of Indexed, the dialog is empty and unusable.) This
is a dockable dialog; see the section on
Dialogs and Docking
for help on manipulating it.

2.4.1. Activating the dialog

from the Tab menu in any dockable dialog by clicking on
and selecting
Add Tab → Colormap.

In the Windows menu, there is a list of
detached windows which exists
only if at least one dialog remains open. In this case, you can raise
the “Colormap” dialog from the image-menu:
Windows → Colormap.

2.4.2. Colormaps and Indexed Images

In an Indexed image, instead of being assigned a color directly (as
happens in RGB and Grayscale images), colors are assigned to pixels by
an indirect method, using a look-up table called a
colormap.

To determine the color that should be shown for that pixel, GIMP looks
up the index in the image's colormap. Each indexed image has its own
private colormap. In GIMP, the maximum number of entries in a colormap
is 256. For a maximum-sized colormap, each index from 0 to 255 is
assigned an arbitrary RGB color. There are no rules restricting the
colors that can be assigned to an index or the order they appear in: any
index can be assigned any color.

It is important to realize that the colors in the colormap are the
only colors available for an indexed image (that
is, unless you add new colors to the colormap). This has a major effect
on many GIMP operations: for example, in a pattern fill, GIMP will
usually not be able to find exactly the right colors in the colormap, so
it will approximate them by using the nearest color available. This is
sometimes referred to as
Quantization.
If the colormap is too limited or poorly chosen, this can easily produce
very poor image quality.

The Colormap dialog allows you to alter the colormap for an image,
either by creating new entries, or by changing the colors for the
existing entries. If you change the color associated with a given index,
you will see the changes reflected throughout the image, as a color
shift for all pixels that are assigned that index. The entries are
numbered with 0 in the upper left corner, 1 to its right, etc.

2.4.3. Using the Colormap dialog

Here are the operations you can perform using this dialog:

Click on a color entry

This sets GIMP's foreground color to the color you click on, as
shown in the Toolbox color area. As a result, this color will be
used for the next painting operation you do.

Ctrl-click on a color entry

This sets GIMP's background color to the
color you Ctrl-click on,
as shown in the Toolbox color area.

Double-click on a color entry

This sets GIMP's foreground color to the color you click on, and
also brings up a Color Editor that allows you to change that
colormap entry to a new color.

Color index

You can select a different colormap entry by typing its index
here, or clicking the spinbutton to the right.

HTML-Notation

This area shows a hex-code representation (such as is used in
HTML) for the color assigned to the currently selected colormap
entry. You can edit the color here, instead of using a Color
Editor, if you want to. See
HTML notation

Edit color

This button (in the lower left corner of the dialog) brings up a
Color Editor that allows you to change the color for the currently
selected colormap entry. The effect is similar to double-clicking
on the entry, except that it does not set GIMP's foreground color.

Add color

This button (in the lower right corner of the dialog) allows you
to add new colors to the colormap. If you click on the button, the
current foreground color, as shown in the Toolbox, will be tacked
on to the end of the colormap. If instead you hold down
Ctrl and click, the background color from the
Toolbox will be added. (If the colormap contains 256 entries, it
is full, and trying to add more will have no effect.)

Tip

If you make a mistake, you can undo it by focusing the pointer in
the image whose colormap you have changed, and then pressing
Ctrl+Z or
choosing
Edit → Undo
in the image menu.

Note

This dialog provides the most commonly used methods for altering the
colormap for an indexed image. The color tools, such as
Brightness/Contrast, Hue/Saturation, etc, do not operate on indexed
images. There are a few plug-ins that do so, including the
“Normalize”, “VColor Enhance”, and
“Stretch Contrast” operations, and it is possible to
create others as well.

Note

If you paint an indexed image with a color which is not in the
Colormap, GIMP will use the most similar color of the Colormap.

2.4.4. The Colormap context menu

Right-clicking on a color in the Colormap selects this color and opens
a pop-up submenu:

Figure 15.15. The Colormap context menu

Edit color

This command opens a color selector which allows you to modify
the color.

Add Color from FG

This command is enabled only if the indexed palette contains less
than 256 colors. The background color of the Toolbox is appended
to the color map.

Add Color from BG

This command is enabled only if the indexed palette contains less
than 256 colors. The background color of the Toolbox is appended
to the color list.